In an article entitled "Ion Implantation of Surfaces" by S. Thomas Picraux and Paul S. Peercy in the March, 1985 issue of Scientific American (at page 102), the authors outline the importance of ion beam implantation technology in the manufacture of Integrated Circuits and in ion-beam modification of metal surfaces. The latter is an emerging technology, while the former is a maturing technology now at the stage of Very Large Scale Integration (VLSI), where, according to the authors, sharply focused ion beams offer much higher resolution than electron beams and visible light. Such sharply focused ion beams would permit defining doped features, without intervening steps of masking, of less than a micrometer across, whose electrical activity might be controlled by as few as 100 dopant atoms.
There are three well defined physical indicia of the source ion plasma and the therefrom derived ion beam that affect controlled implantation and sharp focusing. They are:
beam ion temperature PA1 plasma potential; and PA1 plasma fluctuations. PA1 Magnetic fields have been used with ion sources for various purposes. One of these is to constrain electrons to paths along magnetic lines which results in greater ion efficiency. In these systems, focusing and shaping of the plasma front from which ions are extracted has been accomplished by the use of shaped extraction electrodes made of materials with various permeabilities. With these devices the extraction geometries are fixed. PA1 (a) generating an ionizing electron beam; PA1 (b) generating ionizable atoms; and PA1 (c) colliding the electron beam with the atoms in an ionizing chamber to produce an ion plasma in an ion beam extraction region; CHARACTERIZED BY: PA1 (d) generating a confining magnetic field, for both said electron beam and said plasma, having a central region of low field strength substantially coextensive with said plasma at said ion beam extraction region, and having a peripheral region of higher field strength surrounding said central region, thereby aiding stability and uniformity of said ion plasma in the extraction region.
One beam characteristic which sums up the effect of the above three indicia is beam emittance.
Beam emittance is a difficult concept to grasp. It is a measure of the uniformity of beam divergence in a chosen cross-sectional plane. Accordingly, it is measured by scanning the beam plane with a small slit and plotting the divergance of the emerging beamlet in milli-radians against the radial displacement in centimeters of the slit from the central beam axis. The generated plot is called a phase space diagram, the area of which, in units of cm-mrads, is the beam emittance. The smaller the emittance, the more orderly is the ion beam. A related beam characteristic is brightness, which is defined as the beam current divided by the square of emittance. This definition expresses the difficulty of generating powerful, high current, continuous (d.c.) beams, that are well ordered in phase space.
Beam ion temperature is a measure of beam disorder and directly affects the ability to focus it narrowly. It is a measure of the random kinetic energy of ions in the cross-sectional plane. A high temperature beam is a fuzzy one. Beam ion temperature is usually given in electron volt (eV) units, 1 eV being equal to 11,600.degree. K.
Plasma potential is the electrostatic potential of the ion plasma in the space charge region, ahead of beam extraction apertures, with respect to the surrounding reference potential. Low plasma potential reduces erosion and sputtering of the apparatus. It also results in lower beam contamination and lower variation in ion energy. The latter directly affects the ion implantation-deptn definition, which is important in semi-conductor processing.
Plasma fluctuations, sometimes referred to as "noise", are rapid variations in the density of the plasma from which the ions are extracted. For best beam quality, the plasma density must be matched to the strength of the electric field which extracts the ions from the plasma. Plasma fluctuations make it impossible to properly match the density and electric field at all times and thus lead to a loss of beam quality and an increase in the time-averaged emittance.
Ion sources of the duoPIGatron type may be conceptually partitioned into three regions: electron beam generation, plasma generation and ion beam extraction. The present invention concentrates on the region between the electron-emitting hot cathode and the ion beam extraction apertures. Apparently minor design changes in that region directly affect the ion plasma prior to beam extraction. Ion beam quality, as expressed by low beam emittance and low beam temperature, is itself a result of the spacial and physical homogeneity and uniformity of the source ion plasma.